Based on the fact that you mention "caste" in your OP, my guess is you are local to India (the only country I am aware of where "caste" has meant something since the start of the 20th century). As a result, I will mention that I am North American and so what works for me might not work for you.
I would agree with a lot of the other answers that this is probably worth dealing with, although with one caveat: It depends on how otherwise friendly the people engaging in the argument are. Personally, I have these types of discussions fairly often with my friends, and sometimes they can get heated, but at the end we are all still friends and there is no hard feelings. At this point, I would get kind of upset if an outsider to my discussion came in and butted their nose in something that didn't concern them, and I would take it as this person just making trouble for me and my friends having whatever discussion we were having.
So if you choose to act on what you saw/heard, be prepared that you might be butting your nose somewhere it doesn't belong, and in the end the people having the discussion originally might be friends and you might be the one who ends up getting looked down on because of it. That said, if you could obviously see that someone was obviously very hurt because of it, then it might be more OK to do something about it.
EDIT: I just reread your OP and noticed you are these peoples' manager. In which case I would be extra careful about what you do; you don't want to be seen as the fun-killer in your team (and yes, sometimes these sorts of discussions can be considered as fun, and team-building exercises amongst your team; aggressively disagreeing with someone can produce respect if executed carefully). If you butt in here, you might be discouraging your team from becoming too friendly with each other, which might come back to bite you later in the form of reduced productivity; people who are friends are likely to work better together than people who are simply "co-workers".